The present invention relates to the field of casting, and more particularly to a pattern for lost-pattern casting, and also to methods of fabricating shell molds, and to methods of casting using such a pattern.
So-called “lost-wax” or “lost-pattern” casting methods have been known since antiquity. They are particularly suitable for producing metal parts that are complex in shape. Thus, lost-pattern casting is used in particular for producing turbine engine blades.
In lost-pattern casting, the first step normally comprises making a pattern out of a material having a melting temperature that is comparatively low, such as for example out of wax or resin, and then overmolding the mold onto the pattern. After removing the material of the pattern from the inside of the mold, whence the name of such methods, molten metal is cast into the mold in order to fill the cavity that the pattern has formed inside the mold by being removed therefrom. Once the metal has cooled and solidified, the mold may be opened or destroyed in order to recover a metal part having the shape of the pattern. In the present context, the term “metal” should be understood to cover not only pure metals but also, and above all, metal alloys.
In order to be able to make a plurality of parts simultaneously, it is possible to unite a plurality of patterns in a single assembly in which they are connected together by a tree that forms casting channels in the mold for the molten metal.
Among the various types of mold that can be used in lost-pattern casting, so-called “shell” molds are known that are formed by dipping the pattern or the assembly of patterns into a slip, and then dusting refractory sand onto the pattern or the assembly of patterns coated in the slip in order to form a shell around the pattern or the assembly, and then baking the shell in order to solidify the slip and thus consolidate the slip and the sand. Several successive operations of dipping and dusting may be envisaged in order to obtain a shell of sufficient thickness prior to baking it. The term “refractory sand” is used in the present context to designate any granular material of grain size that is sufficiently small to satisfy the desired production tolerances, that is capable, while in the solid state, of withstanding the temperature of the molten metal, and that is capable of being consolidated into a single solid piece by the slip during baking of the shell.
In order to obtain particularly advantageous thermomechanical properties in the part produced by casting, it may be desirable to ensure that the metal undergoes directional solidification in the mold. The term “directional solidification” is used in the present context to mean that control is exerted over the nucleation and the growth of solid crystals in the molten metal as it passes from the liquid state to the solid state. The purpose of such directional solidification is to avoid the negative effects of grain boundaries within the part. Thus, the directional solidification may be columnar or monocrystalline. Columnar directional solidification consists in orienting all of the grain boundaries in the same direction so that they cannot contribute to propagating cracks. Monocrystalline directivity solidification consists in ensuring that the part solidifies as a single crystal, so as to eliminate all grain boundaries.
Directional solidification is particularly desirable when producing parts that are to be subjected to high levels of thermomechanical stress, such as turbine engine blades. Nevertheless, the complex shapes of such blades can interfere with directional solidification, giving rise to unwanted grains, in particular in the proximity of sharp corners in the blade. In particular, in a turbine engine blade with a root and a body on either side of a platform extending substantially perpendicularly to a main axis of the blade, said body presenting a pressure side, a suction side, a leading edge, and a trailing edge, the sudden transition between the body of the blade and the platform can cause such unwanted grains to form, in particular in the vicinity of the trailing edge.
In order to reduce the weight of turbine engine blades, and above all in order to enable them to be cooled, it is common practice to embed refractory cores in the non-permanent pattern. Such a refractory core remains inside the shell mold after the material of the pattern has been removed, and after the metal has been cast and allowed to cool, thereby forming a hollow volume in the metal part. In particular, in order to provide good cooling of the trailing edge, given that its small thickness makes it particularly vulnerable to high temperatures, it is common practice for such a core to be flush with the surface of the pattern at the trailing edge so as to form a cooling slot for the trailing edge. Nevertheless, the small thickness of the core in this location makes it fragile. In addition, in order to keep the core in the correct position inside the shell mold during casting and cooling of the metal, it is desirable to guide its thermal expansion. For this purpose, the pattern may include a guide strip adjacent to the trailing edge and having a varnished surface of the refractory core flush with each side of the pattern between the trailing edge and the expansion strip. The varnish on these surfaces, which may be removed from the shell mold together with the material of the pattern, ensures that there is a small amount of clearance (of the order of a few hundredths of a millimeter) between the refractory core and the shell mold, so as to guide the expansion of the core at this location in a direction perpendicular to its thickness. Inside the expansion strip, the core can be of greater thickness, thereby making it more robust.
Nevertheless, the complexity between the shape of the mold cavity at the intersections of the trailing edge or the expansion strip with the blade platform significantly increases the risk of grains being generated.